He's a D. Defensemen who make mistakes frequently see those mistakes end up in the back of the net. And if they don't end up in the back up the net, it's probably because the goalie or someone else bailed him out, however the end result is the same - those mistakes are very very obvious. What sucks even more is Maatta isn't the most fleet of foot. Him improving on his turns and first step would make a massive difference - both of these are things he can realistically improve. But with the eye test, it makes things look worse when you see a game end with Ryan powering past him and scoring.
Overall he's significantly better than the eye test says. Especially with regards to his SAT numbers. The fact is a D's job is to defend. One way we try to measure that is with SAT numbers (with the idea that the better the % the less time they're spending in their zone). But when you look at how effective he actually was at helping his team keep the puck out of his net, his numbers were the best on the team. Which means that regardless of what his SAT numbers say, he's still doing his job better than everyone else. And it's not sacrificing offense to do it. The only D with significantly better GF were Schultz and Cole - Letang would have been up there as well. His GF60 was 4th at 3.04.
Letang GP: 41, GA: 31, GA60: 2.54
Schultz GP: 78, GA: 48, GA60: 2.23
Dumoulin GP: 70, GA: 53, GA60: 2.67
Cole GP: 81, GA: 48, GA60: 2.15
Daley GP: 56, GA: 40, GA60: 2.84
Maatta GP: 55, GA: 28, GA60: 2.03
Ruhwedel GP: 34, GA: 16, GA60: 1.93
On a side note, Hainsey had amazing numbers, but I excluded him due to sample size.